The invention relates to frequency planning for wireless networks.
To meet the growing demand for wireless data services, many companies have started deploying wireless local area networks (WLANs) in airports, hotels, convention centers, coffee shops and other locations in which network access by the public is desirable. Many of these WLANs support the popular IEEE standard for wireless Local Area Network (LAN) protocol, known as the IEEE 802.11 standard. The IEEE 802.11 standard includes a medium access control (MAC) layer and several physical layers, including a frequency-hopping spread spectrum (FHSS) physical layer and a direct sequence spread spectrum (DSSS) physical layer. Versions of the IEEE 802.11 standard include the IEEE 802.11a standard, which describes a physical layer based on orthogonal frequency division multiplexing (OFDM), and the IEEE 802.11b standard, which specifies a high-rate DSSS layer. Because of its maturity and low cost, IEEE 802.11b capability has been included as standard equipment in many laptop computers and hand-held devices. Thus, IEEE 802.11b products make up the bulk of the installed base of IEEE 802.11 systems. The IEEE 802.11 WLANs support data rates up to 11 Mbps, albeit over short ranges, far exceeding that to be offered by the third generation (3G) cellular wireless networks.
The IEEE 802.11 WLANs and 3G networks (or conventional cellular wireless networks) have major differences in their design at physical (PHY) and medium access control (MAC) layers to meet different needs. In general, the IEEE 802.11 design is much simpler than that of the 3G network because the IEEE 802.11 standard was devised to serve a confined area (e.g., a link distance of at most several hundred meters) with stationary and slow-moving users, while the 3G specifications were developed for greater flexibility in terms of geographical coverage and mobility, even providing for users traveling at a high speed. As a result, the IEEE 802.11 network can support data rates higher than those by the 3G networks. In addition, the cost of IEEE 802.11 equipment is much lower than that for 3G equipment because of the simple and open design of IEEE 802.11 networks, coupled with competition among WLAN vendors.
In terms of operations, the 3G spectrum (such as the Personal Communications System (PCS) band at 1.9 GHz) is licensed and very expensive. As a result, every effort has been directed toward optimizing the spectral efficiency while maintaining the quality of service in terms of coverage and data rate for a limited spectrum allocation. In contrast, the IEEE 802.11b networks operate in the unlicensed Industrial, Scientific and Medical (ISM) band at 2.4 GHz. Since the frequency band is free, there is apparently no pressing need to optimize the spectral efficiency. Rather, simplicity and achieving low cost for the equipment are more important. Despite the relatively abundant spectrum (i.e., a total of 75 MHz in the 2.4 GHz Band) at the ISM band, as IEEE 802.11b networks are deployed widely, they start to interfere with each other. Such interference leads to a degradation in network throughput.
Frequency planning, i.e., allocation of a limited number of frequencies, for an IEEE 802.11b network is different from that for a traditional cellular network. Frequency planning techniques for cellular wireless networks are well known. In typical cellular wireless networks, such as those based on the Global System for Mobile Communications (GSM) and Enhanced Data GSM Evolution (EDGE) standards, two separate radio channels, namely the traffic and control channels, are used to carry user data and control traffic, respectively. For example, terminals access the control channels to send control information via some contention mechanism. After the information is successfully received and processed by a base station (BS), the terminal is assigned with a specific traffic channel for transmitting its data traffic. Existing frequency assignment or radio-resource allocation schemes were devised mainly for such traffic channels. Such schemes seek to avoid mutual interference among various terminals or BSs using the same frequency. In practical networks, there is no real-time coordination among BSs in the assignment of traffic channels to terminals in different cells. Thus, frequency assignment or radio-resource allocation is based on statistical averages or worst cases, e.g., 90% chance of acceptable link quality, across multiple co-channel cells. Typically, frequency planning mechanisms for traditional cellular networks tend to assign the same frequency to cells that are a sufficient distance apart.
There is no such distinction between control and traffic channels in the IEEE 802.11b network. Instead, all user data and control information (in both directions between terminals and APs) are carried on the same physical channel. The access to the channel by multiple transmitters is coordinated by the MAC protocol, e.g., the well-known, Carrier Sense Multiple Access (CSMA) protocol with collision avoidance feature. Under that protocol, a transmitter can transmit only if it senses that the channel is currently idle. As a result, even if two closely located APs are allocated with the same frequency channel, much of the mutual (co-channel) interference can still be avoided by the CSMA protocol, and the available bandwidth is shared implicitly between the two cells served by the two APs. In a sense, the MAC protocol provides an effective, distributed mechanism to “coordinate” the channel access among terminals and APs. In the worst case, both APs behave as if they share the same frequency. Nevertheless, the IEEE 802.11 protocol still works properly, thus demonstrating the robustness of its design, at the expense of increased delay (due to backoff when sensing channel busy) and degraded network throughput.
Consequently, existing frequency allocation mechanisms that do not consider the combined effect of physical channel and MAC protocol are not directly applicable to the IEEE 802.11 networks. The MAC CSMA protocol helps to avoid much of co-channel interference in large multi-cell IEEE 802.11 networks, but does so at the potential expense of network performance.